A highly selective ESIPT-mechanism-based, ratiometric fluorescent sensor for zinc ions

Zinc is crucial for human health, a fact that has prompted interest in developing efficient fluorescent probes for its detection. Traditional approaches, however, face issues such as environmental interference and limited selectivity. This has led to research focusing on ratiometric sensing and making use of the excited-state intramolecular proton transfer (ESIPT) effect. Tetramic acid and its 4-methoxy analogue are significant structural motifs in many natural products from various organisms, including those which can bind metal ions, such as zinc. This study presents a new ratiometric fluorescent sensor for zinc that leverages ESIPT for high selectivity and efficacy, and combines tetramic acid and pyridine substructures. Our probe exhibits a strong emission band at 515 nm upon excitation in polar aprotic solvents, and upon excitation at 360 nm a selective ratiometric response to Zn²⁺ in acetonitrile, with distinct, blue-shifted emission maximum at 460 nm. This is accompanied by a color change from green to turquoise, which is visible to the unaided eye under a UV lamp. Our sensor shows excellent sensitivity, which is confirmed by a low detection limit of 1.26 × 10⁻⁶ M. The binding of the compounds to Zn²⁺ ions was further confirmed in an NMR study, and the effects of common anions and change in pH were also examined. The overall simplicity of our structure, alongside its unique properties and open possibilities for structural modifications, makes it attractive for further research.